I heard in one place that it was because this is the best way to mix them well. I heard somewhere else that this is a carry-over from when flour used to still contain some chaff. What's the real reason? When does one still need to do this?

5 Answers
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Fix Compacted Flour. Flour will compact over time (and during shipment). You could sift the flour to fluff it back up. Or, you could just stir it before measuring and be sure to spoon the flour into your measuring cup in order to get a correct volume measurement.

Remove Unwanted Material. Yes, sifting would also remove larger pieces or bits of chaff. It would also remove insects. However, neither of these are problems with modern store-bought flour. If you grind your own flour (my sister-in-law does this), then you may still want to sift it, though.

Mix Ingredients Together. Sifting can also be used to mix other dry ingredients into flour. I bake quite a bit, though, and I've never had a problem with just using a spoon or whisk to mix dry ingredients together.

In summary: don't bother. Just use good measuring techniques and stir your ingredients together well. No one likes a lump of baking powder lurking in a muffin!

I was baking once at my mom's housel, and the pantry was in her garage and subject to humidity and temperature swings. Unfortunately, bugs had gotten into the flour, and it was the last bag, so we sifted ... So occasionally it's still necessary, if you don't store things correctly. She didn't have a dedicated sifter, we just used a mesh strainer.
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JoeAug 16 '10 at 23:52

Sifting aerates the flour. This alters the texture of the finished good, resulting in a lighter, airier texture. This still has relevance today, and should be done when the recipe calls for it, but you can experiment freely on your own. Generally there is no need to sift when making bread, biscuits or scones. Delicate sponge or chiffon cakes using pastry or cake flour must be sifted given the proclivity for clumping these flours have.

You should be aware that pulsing your dry ingredients in a food processor is a bit more effective at aerating, as well as mixing the ingredients. It also requires much less effort.

I use a whisk in a bowl to mix dry ingredients before adding them to wet ingredients when there's a huge difference in the volume between some of them. For example, adding a few teaspoons total of soda, salt, spices, etc., to two cups of flour. I think this does a good job of distributing the ingredients and I don't care much for using an actual sifter.

Also, for recipes that call for sifting flour, etc., before measuring by volume, you will get a vastly different amount of that ingredient compared to measuring without sifting it. You can cheat this by using weight instead of volume.

Sifting flour or cocoa powder is a chore, but sometimes it is
important. When making a delicate cake like a sponge cake or genoise
that requires flour to be folded into beaten eggs and sugar, sifted
flour can be added quickly and distributed evenly (because sifting
aerates the flour), thereby reducing the risk of deflating the batter.
Recipes with cocoa powder, such as chocolate cake, also often call for
sifting the cocoa powder. In this case, sifting breaks up small clumps
of cocoa that form as the powder sits in the package. Sifted cocoa can
be evenly distributed throughout a cake batter; with unsifted cocoa
this isn’t always the case.